<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        
    </style>
</head>
<body>
    
</body>
<script>

    var a;
    console.log(a);
    a = 10;
    console.log(a);
    // a = a + 2;
    // a += 2;
    // a -= 2;
    // a *= 2;
    // a /= 2;
    // a %= 2;
    console.log(a);
    
</script>
</html>